The sealing of adjacent valve surfaces against fluid flow is commonly accomplished by means of a seal ring disposed within a groove or channel formed in one of the valve surfaces. Valves generally have some form of retention assemblies for retaining the seal in the sealing surface. Such retention assemblies, however, not only increase the cost of the valve but, under certain operating conditions, can actually compromise the valve's performance. This is especially true for valves which are required to withstand extreme conditions of temperature and pressure such as, for example, in High Performance Butterfly Valves (HPBV). The subsequent discussion is directed to specific applications in HPBV as being exemplary of other valves in the art and is not intended to be a limitation on the application of the invention.
Various designs have been used for retaining the seal members within a groove in a sealing surface of the valve even under adverse operating conditions. An example is shown in U.S. Pat. No. 3,642,248, issued to Robert E. Benware and owned by the assignee of the present invention. Benware discloses a valve sealing mechanism wherein a seal ring is confined in an undercut circumferential groove. These seal rings are usually composed of a resilient material having the ability to deform and reform to their original shape. They are provided with a pair of longitudinal shoulders which are engagable with opposing flanges formed in the circumferential groove.
An optional backing ring may be used to force the shoulders of the seal ring to abut the flanges formed in the groove. This backing ring serves the dual function of urging the seal ring out of the groove into firm contact with the sealing surface and of sealing against leakage of fluid under pressure behind the seal ring.
Installation of seal rings within a groove has heretofore been accomplished in different ways. For example, installation can be accomplished by temporarily deforming the seal ring and inserting it between the retaining flanges and into the groove. Once in place, it reforms and is retained in the groove by the retaining flanges abutting or aligning with the longitudinal shoulders of the seal ring.
It will be understood, however, that if the seal ring is designed so that it can be readily inserted into the groove, then it is possible for it to be pulled out when the seal ring is placed under severe operating conditions by high velocity fluids. This possibility of extraction is caused by a low pressure region created as the fluid flows past the seal ring at high velocity. The low pressure region creates a vacuum over the seal ring and acts to draw it into the flow stream. Theoretically it might be possible to design a seal ring so that it seats into a groove sufficiently tight to prevent it being drawn out by the low pressure region. However, this would require special equipment to install the rings and there is an increased risk of damage to the ring during installation.
Valves also have been provided with separate seal retaining ring assemblies which either cover the full raised face of the valve or are the screwed-on type which partially cover the valve face. These assemblies generally permit much closer clearances between the retaining rings and the seal rings and, further, permit the use of non-deformable seal rings of various compositions. They do, however, possess the disadvantage of having an inherent leak path created by the separate retaining ring.
In the case where a separate full face cover is provided, the leak path to the outside of the valve is formed at the interface between the retaining flange and the sealing face of the valve housing. To block this leak path, separate gasket materials can be incorporated between the retaining ring and the valve body. In the screwed-on type retaining ring assembly, the creation of the leak path is further compounded by the attachment screws which interrupt the sealing face of the valve. This interruption increases the possibility of flange gasket failure.
Where a separate retaining ring assembly is used, installation and/or replacement of seal rings is complicated. The retaining ring and gasket seal must be removed, usually necessitating complete removal and disassembly of the valve.
In each of the methods discussed above, particular consideration must be given when the valve is used under dead-end service conditions. These conditions occur when a valve is placed under pressure on one side of the sealing surface while the piping is removed from the opposite side. Where this occurs, the retainer assembly is exposed and must support a portion of the extreme pressure exerted on the valve sealing surface.